Many pizza box type enclosures include a removable fan shelf module that provides air flow and cooling to electronic components within the enclosure, such as mounted in various compartments or shelves. This pizza box type enclosure is also referred to as a pizza box form factor because it is a style of computer case that typically is very thin, for example, a one-rack unit or two-rack unit (1U or 2U) and typically about 1¾ or 3.5 inches in height, making them wide and flat, thus, looking like pizza delivery boxes. Pizza box type enclosures are often used in data centers because of the stackability of that type of system, where servers must be periodically added or enlarged with various components. Some of these systems using this type of pizza box type enclosure include higher end Ethernet switches for mounting at the top of racks. Typically, these pizza box type enclosures include a chassis and have electronic components installed and mounted in different orientations. For example, sometimes an electronic component or other unit is installed both horizontally and vertically.
In a horizontal unit, there is no real benefit in blowing the air from the right to the left or the left to the right unless there is an obstacle impeding the air flow in one direction or an air heating element located on one side or the other. For a vertically mounted unit, natural convection provides air flow upward in a vertical direction. It is possible, however, that the preferred installation direction is opposite to this natural flow because of cable routing considerations or other factors. For example, one service provider created this type of problem in a remote enclosure installation using the Total Access 1248V as manufactured by ADTRAN, INC. In that installation, the cooling fans are directing the air flow downward. Other installations for outdoor remote enclosures have been known to be vertically oriented and have fans that blow in a direction opposing the air flow of the pizza box enclosure. In these cases, the air flow and thus the cooling are dramatically reduced due to the fans flowing in opposing directions. This reduces the air flow velocity in the box, resulting in overheating of components.